Ring laser gyros have a closed gas-laser path formed by three or more reflective corner mirrors. Two or more optical beams counterpropagate in the optical path. As the gyro is rotated about a sensing axis, the frequencies of the counterpropagating light beams vary. By sensing the variation of the light frequencies, a signal is produced that is a measure of the angular velocity to be measured. To sense the variation of the light frequencies, and more particularly to sense the differences between the light frequencies of two beams, it is necessary that an optical system be provided to extract a portion of the light from each beam.
Some ring lasers are multisensors which use multioscillators. That is, they have more than two beams, and the beam frequencies vary in a particular way in response to rotation about the sensing axis of the multioscillator.
A gyro multisensor which uses a differential four-mode ring laser gyro, herein called DILAG multioscillator, there are usually four beams, all elliptically polarized, including DILAGs, are described in, "Multioscillator Laser Gyros" by Weng Chow et. al, IEEE Journal of Quantum Electronics, QE16-9, September 1980, pages 918-936. It typically uses a four mode ring laser having two non-reciprocally biased ring laser beam pairs.
In a four-mode ring laser gyro for a DILAG multioscillator, four beams circulate within a ring laser, and the laser path is, typically, a non-planar closed loop having a sensing axis defined within the loop. The apparatus measures angular velocity, W, or displacement of the gyro about the sensing axis.
Usually, four corner mirrors define a non-planar closed path. Two oppositely elliptically polarized beams propagate in one direction, and two oppositely elliptically polarized beams propagate in the other direction around the closed loop. The individual beams are each very close to circularly polarized, and they are further recited herein as circularly polarized.
Two pairs of beams, each pair having a beam propagating in each direction around the laser path, are defined. Number the four beams 1, 2, 3, 4 and the corresponding frequencies of the four beams, from the lowest to the highest frequencies, f.sub.1, f.sub.2, f.sub.3, f.sub.4. Due to the configuring of the laser path into a non-planar path the pairs of beams having frequencies f.sub.1, f.sub.2 and f.sub.3, f.sub.4 are reciprocally biased upwards and downwards in frequency from gain center frequency, f.sub.o.
In each pair of beams, the frequencies of the two beams are nonreciprocally biased upwards and downwards in frequency to separate the two frequencies in each pair. Beams 1 and 4 propagate in one direction through the ring laser path, and beams 2 and 3 propagate in the other direction along the ring laser path.
The frequencies f.sub.1 and f.sub.2 form a first gyro, and the frequencies f.sub.3 and f.sub.4 form a second gyro. The sensing axis of the DILAG multioscillator gyro is predefined within the laser path loop.
As the DILAG laser is rotated about its sensing axis in a first direction, the frequencies f.sub.1 and f.sub.2 move farther apart, and the frequencies f.sub.3 and f.sub.4 move closer together. When the angular velocity, W, of the laser about its sensing axis is reversed, the frequencies f.sub.1 and f.sub.2 move closer together, and f.sub.3 and f.sub.4 move farther apart. The angular velocity, W, is proportional to the frequency difference f.sub.1,2 between the first and second frequencies f.sub.1 and f.sub.2, minus the frequency difference f.sub.3,4 between the third and fourth frequencies, f.sub.3 AND f.sub.4.
Prior art optical systems for extracting light from the DILAG multioscillator use a quarter waveplate having a nominally zero degrees orientation with a polaroid sheet-polarizer. A polaroid sheet-polarizer has a 35% absorption loss. Polaroid sheet-polarizers warp with changes in temperature. Further, such polarizers are damaged by the high temperature processing required to manufacture a typical ring laser gyros in which they are imbedded.
In a typical DILAG multioscillator, two oppositely elliptically polarized beams propagate in a clockwise direction around the ring laser gyro path. Two other oppositely elliptically polarized beams propagate in a counter-clockwise direction around the ring laser gyro path. The laser path is, typically, defined by four corner mirrors forming a non-planar closed loop having a sensing axis defined within the loop. The apparatus measures angular velocity, W, or displacement of the gyro about the sensing axis.
The individual beams are almost completely circularly polarized, and they are herein further described as circularly polarized.
Two pairs of beams, each pair having a beam propagating in each direction around the laser path, are defined. Number the four beams 1, 2, 3, 4 and the corresponding frequencies of the four beams, from the lowest to the highest frequencies, f.sub.1, f.sub.2, f.sub.3, f.sub.4. Due to the configuring of the laser path into a non-planar path the pairs of beams having frequencies f.sub.1, f.sub.2 and f.sub.3, f.sub.4 are reciprocally biased upwards and downwards in frequency from gain center frequency, f.sub.o.
In each pair of beams, the frequencies of the two beams are nonreciprocally biased upwards and downwards in frequency to separate the two frequencies in each pair. Beams 1 and 4 propagate in one direction around the ring laser path, and beams 2 and 3 propagate in the other direction around the ring laser path.
The frequencies f.sub.1 and f.sub.2 form a first gyro, and the frequencies f.sub.3 and f.sub.4 form a second gyro. The sensing axis of the DIIAG multioscillator gyro is predefined within the laser path loop.
As the DIIAG laser is rotated about its sensing axis in a first direction, the frequencies f.sub.1 and f.sub.2 move farther apart, and the frequencies f.sub.3 and f.sub.4 move closer together. When the angular velocity, W, of the laser about its sensing axis is reversed, the frequencies f.sub.1 and f.sub.2 move closer together, and f.sub.3 and f.sub.4 move farther apart. The angular velocity, W, is proportional to the frequency difference f.sub.1,2 between the first and second frequencies f.sub.1 and f.sub.2, minus the frequency difference f.sub.3,4 between the third and fourth frequencies, f.sub.3 AND f.sub.4.
Combining optics are needed to extract a portion, usually about 0.01% of the beam, from the ring laser. The four beams, upon exiting the multioscillator gyro, traverse two separate pathways through the combining optics, including at least one prism, to impinge on a first detector, and they traverse another two separate pathways to impinge upon a second detector.
In the prior art, at the output of the combining optics is one common quarterwaveplate and two suitably oriented polarizers in front of each of the detectors to receive the beat signals from the left and right circularly polarized gyros. In an ideal situation wherein all the phase shifts experienced by all the beams within the combining optics for both s and p polarizations are equal, the crosstalk approaches zero. When the various phase shifts are not all equal, the minimum crosstalk is a strong function of the deviation of the phase shifts from some common value.
The s component of polarization is defined perpendicular to the p component of polarization. For discussion, the p direction shall be considered to be parallel to the paper, and the s direction shall be considered normal to the paper.
The helicity of polarization, for each mode, alternates going from leg to leg of the ring laser. Because of the alternating of helicity direction, the designation "right handed" and "left handed" must be taken relative to a particular leg of the gyro.
It is determined that modes L1 and R4 enter the combining optics from the clockwise, or "C" direction, and modes R2 and L3 enter the combining prism from the anticlockwise, or "A" direction.
The L1, R2, L3, R4 received beams are changed to L1, L2, R3, R4 beams, whereby the L1 and L2 beams form a first gyro and the R3 and R4 beams form a second gyro. The reversal of ellipticity of beams R2 and L3 to L2 AND R3 is achieved by causing those beams to undergo an odd number of reflections while the L1 and R4 beams undergo an even number of reflections within the combining optics. If the total birefringence effects, up to the interface with a quarter waveplate, are kept to a minimum, the sequence L1, L2, R3, R4 beams are delivered to the quarter waveplate. The p-to-s amplitude ratios of the elliptically polarized beams within the laser are substantially one-to-one. At any point within the combining optics, it increase substantially, for example ten to one, because of the preferential transmission of p polarized light at the laser corner mirror. The quarter waveplate produces linearly polarized light beams along two non-orthogonal directions. In the ideal situation, the beat frequency component between beams 1 and 2 can be totally extinguished, by an ideal polarizer, with a transmission axis suitably placed, at one detector, and the beat frequency component between beams 3 and 4 can be totally extinguished, by an ideal polarizer, with a transmission axis suitably placed, at the second detector.
The prior art devices used polaroid sheet-polarizers in their apparatus which proved inefficient and difficult, if not impossible, in the heat environment needed to process commercial grade ring lasers.